The present invention relates to semiconductor integrated circuits, and more particularly to a circuit for clamping the voltage received by an n-type region formed in a semiconductor substrate.
One conventional technique for ensuring that the voltage applied to an n-type semiconductor region does not fall significantly below the ground potential, is to place a Schottky diode between the n-type region and the ground, and further, to place a current limiting resistor between the n-type region and the node that may pull the n-type region below the ground potential, as shown in FIG. 1. As node 16 is pulled below the ground potential, Schottky diode 12 is forward biased thus maintaining n-type region 10 clamped at a forward Schottky diode voltage below the ground potential.
One disadvantage of the clamping circuit shown in FIG. 1 is that Schottky diode 12, which is a metal-semiconductor junction may not be available for use. Second, if the Schottky diode has a relatively high series resistance and/or a high forward voltage, n-type region 10 may be clamped at a voltage sufficiently below the ground potential as to cause an associated parasitic lateral NPN transistor to turn on.